This invention relates to a method and apparatus for the casting of molten metals as continuous strip and, more particularly, to the casting of lead and wide-freezing range lead alloys as continous strip for use as electrode grids for batteries.
For many years, the manufacturers of lead-acid batteries have used a variety of lead alloys in the preparation of grids. The casting methods for these alloys include book mold casting, casting into a slab followed by rolling to form a wrought product in the form of a strip, belt casting, twin-belt casting, double-drum casting, and casting onto a drum rotating in a bath of molten alloy, the so-called "melt extraction solidification" or "dip-casting" method. The last-mentioned method allows the manufacture of alloy strip directly from molten alloy.
Successful dip-casting involves the providing of a smooth flow of dross-free molten metal to a zone through which the circumference of a chilled casting drum rotates. It is necessary to extract heat at a uniform rate across the width of the strip to produce a uniform thickness of the strip across the drum. The dip-casting method is suitable for casting pure lead and for casting lead alloys which possess a narrow freezing range, such as lead-calcium or lead-calcium-tin alloys. For the manufacture of battery grids from cast strip, the lead alloy strip is expanded and shaped to form the mesh used for battery grids. Other methods used for producing grids have included the direct casting of alloy in the form of a grid and the casting of grids using a rotating drum with a surface shaped to correspond with the desired grid shape.
Both the alloy compositions and the methods of casting are the subject of numerous patents. One successful process for casting lead or lead-calcium or lead-calcium-tin alloy strip using the melt extraction solidification method is disclosed in U.S. Pat. Nos. 3,926,247 and 3,858,642, while the expanding and shaping of the cast strip for the manufacture of lead-acid battery grids is disclosed in U.S. Pat. Nos. 4,291,443, 4,297,866 and 4,315,356.
Currently, many automotive battery manufacturers favour the use of low antimony-lead alloys for the positive electrodes grids in maintenance-free batteries. These manufacturers claim that the low antimony-lead alloys provide longer battery life as compared to other lead alloys such as lead-calcium alloys. Low antimony-lead alloys for positive battery plates generally contain from 0.5% to 4.0% Sb. For automotive starting batteries, the alloys usually contain from about 1.0% to no more than about 2.5% by weight of antimony. Below about 1.0% Sb, battery grids made from such alloys have reduced deep cycling capabilities. To enhance the castability, and the mechanical and electrochemical properties of the lead-antimony alloys, one or more additional alloying elements are usually added. These additional alloying elements include arsenic, copper, tin, sulfur, selenium, tellurium, silver, cadmium, bismuth, calcium, magnesium, lithium and phosphorous in amounts ranging from 0.001% to 0.5% by weight of the lead. Many of the additional alloying elements, such as sulfur, copper, selenium, tellurium and silver are added as grain refiners.
The industry, considering that one or more grain refiners are necessary to obtain battery grids with a satisfactory structure and performance, has to a large extent adopted their use. As a result, one or more of these grain refiners are now present in most of the low antimony-lead alloy compositions.
When the low antimony-lead slab cast alloys are made into battery grids by rolling to, for example, 10% of the original slab thickness, the grids made from the wrought strip product have not exhibited satisfactory life performance when used as positive electrodes due to poor corrosion resistance and undesirable grid growth, and therefore this product is not in commercial use. Currently, the positive battery grids accordingly are made by gravity casting (also known as book mold casting) methods and are relatively thick and heavy, have a porous and non-uniform micro-structure which promotes corrosion, can be subject to grid growth, and cause high water loss in a battery. All these characteristics shorten the battery life. The gravity casting method, however, appears to be the only method that is used on a commercial scale to make positive low antimony grid electrodes.
It is shown in the prior art that low antimony-lead alloys for grids of maintenance-free lead-acid batteries may be cast by dip-casting on a rotating drum, by casting on a rotating drum having a grid-shaped surface, by die casting in a mould having a grid-shaped configuration of mould cavity, or by gravity casting and stamping (e.g., U.S. Pat. Nos. 3,789,909, 3,789,910, 4,455,724 and 4,456,579). The applicants herein have attempted to produce strip by dip-casting but such attempts have been unsuccessful and to date this method is not being used in industry. Similarly, the casting on a rotating drum having a grid-shaped surface has not been commercialized for positive plates because severe problems occur with the performance of batteries that contain positive plates made of low antimony lead cast by this process.
Low antimony-lead strip may be cast using the twin-roll casting method and controlling the temperature immediately after rolling in order to provide a homogeneous fine crystalline structure (U.S. Pat. No. 4,498,519). It is known that wrought antimonial lead alloys are inherently soft and that heat treatments are required to harden the alloys so that they become suitable for the manufacture of battery grids. Various heat treatment methods which include quenching or cooling, and aging steps are described in U.S. Pat. Nos. 1,674,954 to 1,674,959; 4,629,516 and 4,753,688. Also, in U.S. Pat. Nos. 4,629,516 and 4,753,688 are disclosed methods for strengthening a lead-antimony alloy by rolling the alloy, heating the alloy to provide a recrystallized structure which strengthens on aging, and quenching the alloy. The tensile strength of the treated alloys is increased. The alloys comprise 0.5% to 6% Sb and 0.002% to 1% As, the balance being lead, and 0.5% to 6% Sb, 0.002% to 1% As and 0.02% to 0.5% Sn, the balance being lead, respectively. The rolling of the alloy produces a wrought strip which is heated and subsequently quenched. Battery grids produced according to these patents, however, are also subject to the problems of corrosion and undesirable growth which shorten battery life. Negative battery plates are currently made from lead-antimony, lead calcium or lead-calcium-tin alloys by gravity casting or by expanding lead-calcium or lead-calcium-tin alloy strip.
Low antimony-lead alloys cannot be cast by dip-casting onto a smooth rotating drum for two important reasons. Firstly, the antimony in the alloy causes the molten alloy to exhibit a wide freezing range of up to 60 Celsius degrees in the preferred range of 1% to 2.5% Sb. Secondly, gravity destroys the continuity of the molten metal on the drum. As a result, a coherent, solid, thin strip of uniform thickness cannot be formed. This is especially so when the alloy contains antimony in the range from 1.0% to 1.5% Sb in which the solidification range of the alloy is at a maximum.
Another method for casting metal alloy strip is the casting onto a cooled, rotating drum from a tundish, casting trough or casting vessel positioned above or onto the side of the drum, the so-called "melt-drag" method. Although the melt-drag method of casting metal strip is used for preparing strip of aluminum, aluminum alloys, copper, copper alloys and steel, to our knowledge, the method has not been used commercially to prepare strip of wide-freezing range lead alloys, such as low antimony-lead alloys.